This invention relates to a cathode composition for improving the stability of high performance solid oxide fuel cells which uses a stabilized zirconia as the electrolyte membrane and (optionally) as one component of the cathode. The high performance of such cells is reduced by reactions between the electrocatalyst (usually a perovskite) and YSZ. This problem is particularly acute when rare earth perovskites of cobalt, known to be excellent cathodic electrocatalysts for solid oxide fuel cells, are used to boost cell performance. For example, lanthanum cobaltite reacts with YSZ to form an insulating layer of lanthanum zirconate, which severely degrades the performance of the cell by inhibiting the flow of electrons and ions within the fuel cell. Other lanthanum perovskites such as manganites and ferrites also react with YSZ but not as readily.
It is known to prepare a solid oxide fuel cell comprising a dense electrolyte membrane of a ceramic oxygen ion conductor, a porous anode layer of a ceramic or a metal or, most commonly, a ceramic-metal composite, in contact with the electrolyte membrane on the fuel side of the cell, and a porous cathode layer of an electronically-conductive metal oxide on the oxidant side of the cell which generates electricity through the electrochemical reaction between a fuel and an oxidant. This net electrochemical reaction involves charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase (fuel or oxygen).
Electrode structures comprising a porous layer of electrolyte particles on a dense electrolyte membrane with electrocatalyst material on and within the porous layer of electrolyte are known. In such electrodes, the electrocatalyst material is continuous on the surface of the porous electrolyte material to create a three phase boundary (TPB) where the electrolyte material, electrocatalyst, and gas are in contact. The electrode is prepared by applying an electrocatalyst precursor material as a slurry to a porous electrolyte structure, and then heating the precursor material to form the electrocatalyst. However, it is usually necessary to repeat the process of applying the electrocatalyst precursor material to the porous substrate several times in order to provide enough electrocatalyst to obtain a fuel cell with the desired performance characteristics. For fuel cell applications, this method of creating the layer of electrocatalyst in and on the porous electrolyte structure by repeated applications of the electrocatalyst slurry may create more process steps in the preparation of the fuel cell than would be desirable in a commercial manufacturing process. In addition, the performance characteristics of the electrode structure prepared by such processes, such as the voltage at a certain current density, may be less than desirable for certain applications.
U.S. Pat. No. 3,377,203 discloses a method for producing fuel cells of solid electrolyte and ceramic oxide electrode layers by sintering the electrode layers to the electrolyte. U.S. Pat. No. 4,767,518 discloses a solid oxide electrode (anode) made of metal particles that are immobilized by stabilized zirconia which may also contain praseodymium (Pr). The Pr may be added in the form of a solution. U.S. Pat. No. 4,885,078 discloses an electrochemical device which may be a solid oxide cell which comprises a porous electrode containing a deposit of metal oxide or metal Basalt capable of forming metal oxide upon heating, where the metal may be Pr. U.S. Pat. No. 5,021,304 discloses a method of coating a separate electronically conducted layer on a porous electrode having the steps of applying a mixture of metal salts including nitrates to the electrodes with a surfactant, and heating to form the oxides. Pr oxide is included in a list of dopant oxides which may be used.
The present invention covers the use of a high performance electrocatalyst, based on transition metal perovskites of praseodymium, samarium, terbium or neodymium, which react with YSZ to form a product which is itself active as the cathode in a fuel cell. While PrCoO3 reacts with YSZ, the present inventors have discovered that the reaction product(s) do not result in severe degradation of cell performance. A fuel cell made with a cathode composed of only the reaction product of YSZ+PrCoO3 has good performance, indicating that this phase is itself not only a good conductor, but also a good catalyst for oxygen activation.
In a fuel cell containing such an electrocatalyst, there is an initial induction period where reaction between YSZ and the electrocatalyst takes place, but after this induction period the fuel cell performance is stable. The presumed structure of the interface between YSZ and the electrocatalyst (PrCoO3) is shown in FIG. 1.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.